Method for primary aluminum smelting integrated with a solid fossil-fuel fired power plant, with choice of technology allowing small size, minimal capital intensity, extra energy production available for a local grid, and competitive operating costs

ABSTRACT

A method of processing aluminum combining an alumina reduction plant operating as per the Hall-Heroult process for electrolysis of alumina of the Vertical Stud Soderberg (VSS) type, with a fluidized bed boiler (FBB) or circulating fluidized bed boiler (CFBB) power plant consuming any solid fuel- or carbon-based fuel to produce aluminum metal in ingots or sows or in liquid form in ladles, and to generate electrical energy which is both used by the smelter itself in the alumina reduction reaction and available for a local energy grid.

BACKGROUND OF INVENTION

1. Field of Invention

This invention refers to methods of aluminum production, specifically an improved method for aluminum smelter organization, leading to smaller size and competitive operating costs.

2. Prior Art

Aluminum is a metal growing in use, with a world consumption of around 28 million tonnes per year (2003) and growing. However one notices that this industry is currently concentrated in only about 250 smelters in the world. This is the case because until now, economies of scale have favored very large production units.

Aluminum production by electrolysis of alumina is very energy intensive whatever the process used. All smelters consume from 14 to 15.5 kWh per kg of metal produced according to the age of the smelter, exact technology used, quality of maintenance, and efficiency of operation. Most of that energy consumed is the reduction energy itself.

Primary aluminum production takes place in around 250 smelters in the world. All use the Hall-Heroult Process originating in 1880, consisting of electrolysis of alumina, or aluminum oxide, mixed with cryolite and other minerals forming an eutectic bath which melts at a lower temperature than pure alumina. In this process, electrolysis of alumina reacting with a carbon anode converts alumina and carbon into liquid aluminum and carbon dioxide. Energy supply is a critical factor since the reaction is very energy intensive: roughly 14,000 kWh is needed to produce one metric tonne of molten metal today. Technology has progressed generally to improve energy efficiency, at the cost of bigger and bigger smelters in which the current intensity and the number of cells has grown from levels of 10,000 Amperes in 1900 to 300,000 Amperes today, and the capacity has grown from a few thousand tonnes/year per plant to 250,000 and even 500,000 tonnes/year today. New investments (called “greenfield plants” in engineering lingo) are rare (one per year to one every two years in average worldwide) due to the growing difficulty in concentrating on one site an energy source allowing for sufficiently low cost per kWh procured, for a period allowing depreciation of the huge capital costs (typically $4,500 to 5,000 per tonne/year capacity in 2000-2004). It is generally recognized that between 2000 and 2004 the world aluminum industry spent on average 2 US cents per kWh or roughly $280 per tonne of aluminum produced.

The need to secure low cost energy has led to favoring of energy sources which allow easiest power concentration in one site: hydro-electric, coal-fired, gas-fired, or sometimes nuclear plants. Because of sheer size of projects, such sites are under more and more scrutiny regarding their pollution potential, meaning that overall costs, including political costs, of environmental impact studies to get a project approved have been escalating. Again because of combination of size of project and of high relative depreciation costs, the costs and delays of financial engineering have been also escalating. It is common that the various site evaluations, viability assessments, preliminary impact studies, pre-feasibility studies and bankable feasibility studies for a greenfield smelter planned in a country which doesn't produce aluminum yet can encompass a period of 10, 15 and even 20 years, sometimes to end in a decision not to pursue. Thus, tens of millions of dollars of such costs have been lost in projects in Chile, Libya, Malaysia, British Columbia, etc. No new smelter has been built in the US since 1969.

Two configurations of smelter pot lines exist:

The first configuration is the so-called Pre-Baked Anode (PBA) (illustrated in Figure A): Up to 40 rodded anodes (1) per cell, produced separately (Anode Plant with Baking Oven and Rodding Shop) are consumed as carbon of the anode and alumina (2) fed from the hopper are reduced in the bath of the cell (3) into carbon dioxide and aluminum. Rodded Anodes are changed in staggered order. Carbon anode blocks are produced through mixing petroleum coke and pitch into an anode paste, then blocks are molded and baked separately, then rodded on a metal rod and stem arrangement, the rodded anodes being connected on a busbar (4) above each cell, and replaced by a new rodded anode when they are consumed to around 80% of their volume and reduced to a minimal anode butt, the anode butt being then recycled.

The second configuration is the so-called Soderberg Smelter (described in Figure B) invented by Dr. C. J. Soderberg in 1927: Anode paste is directly fed in a “sleeve” or “shell” (5) above each cell forming a single Anode per cell (6). The paste, as the anode consumes continuously, is continually baked in (7). Anode paste is simply loaded on top (8), held by staggered studs (9) which are added, and pulled out, one after the other. There is no disruption of the process caused by anode change as in the PBA process.

Note that the heat generated by the cell progressively bakes the anode, which is continuously consumed and renewed and doesn't undergo a reheating as in the PBA process.

The Soderberg process gained market share and recognition because of its simplicity, while progress was made in energy efficiency in both processes between 1927 and 1972.

In the sixties, the invention by Aluminium Pechiney of so-called Magnetic Compensation combined with computerized process control monitoring alumina feeding, anode change, anode height, and other parameters gave an advantage to the PBA technology as shown in Figure A. This eventually lead to Soderberg shown in Figure B being abandoned for greenfields, even though similar improvements, including Magnetic Compensation, were later introduced with success on certain Soderbergs. Soderbergs were also criticized for their higher level of pollution. However, recent improvements have brought the best Soderbergs to the same level as PBA's. In 2004 Soderbergs still represent some 20% of world capacity, and extensions (added potlines) of Soderberg plants are still occurring, notably in Brazil. Any new investment in a greenfield smelter faces the following problems which our invention addresses:

(a) Minimal investment is of some 250,000 metric tonnes per year (tpy) capacity, needing a constant power supply of around 600 MW.

(b) Capital cost is at least $4,500 to $5,500 per tpy without energy generation.

(c) If energy generation is added, investment costs go from $500-$650 per kW for natural gas (but gas prices are high and volatile), to $1,000-$1,200 for conventional pulverized coal boilers using the purest quality of high caloric content coal, to $2,000 for nuclear plants, to $2,500 and well over for geothermic and hydro electric plants, but economies of scale generally lead to minimal power of around 1,200 MW per plant. Total capital cost escalates to well over $2 billion. In practice, greenfields are today built close to existing hydro-electric capacity (Quebec, Mozambique, Iceland, Venezuela . . . ) or to oil fields generating excess gas (Middle East). Availability of sites is more and more a constraint.

(d) Separate baking ovens for anodes consume extra power, generally from burners consuming costly, highly refined hydrocarbons.

(e) Prebaked Anode material is heated twice: during baking, and again when inserted in the cell bath. Rodded anode butts need to be recycled. The whole prebaked anode process imposes a costly investment in continuous handling, butt removal, crushing, stem straightening, yoke straightening, re-rodding.

A consequence of these growing inconveniences is that the price per tonne of aluminum measured in relation to the price per tonne of carbon steel, which had been regularly getting more competitive between 1900 and the late 1960's, plateau'ed around 1970-74 and since then has lost competitiveness. See Figures C-a and C-b. Figure C-a illustrates that, since 1910 until the early 70's, aluminum became more price competitive against steel. Figure C-b illustrates that since 1972 the trend has been reversed, with steel becoming more competitive.

Production costs of aluminum being highly energy intensive, the world industry has of course always favored sites were energy costs can be low in the long term. The world industry, in 2000-04, pays an average of 2 US cents per kWh, with actual cost ranging from as low as 0.5 cents (Quebec, Mozambique) to as high as 3 cents and more (most US and European smelters). A crucial issue when deciding where and when to build a new smelter is the long term contracts guaranteeing a low energy cost for at least the years, which is the period of high depreciation costs. Obtaining such long-term contracts, favorable rates, on such high total volume of energy per year is the factor which is delaying more and more all greenfield smelter projects in the world. For instance, Malaysia has been attempting to fund a smelter since 1988; Chile and Qatar since the mid 80's; Guinea, since 1937. In a highly favorable country such as Iceland a greenfield smelter takes a decade or more to go from initial assessments to start up.

BACKGROUND OF INVENTION—OBJECTS AND ADVANTAGES

This combination of a Soderberg aluminum smelter combined with a fluidized bed boiler (FBB) power plant has several economic benefits over mainstream design of power plants using either nuclear fuel, hydro-electric energy, geothermal energy, petroleum gas, or high grade coal burned in pulverized form, and of aluminum smelters using the conventional PBA technology:

-   -   a) Small size of plant (¼th of conventional smelter);     -   b) Lower capital intensity: $2,600-$3,000 for the Soderberg         smelter, $800-$950 per kW for the fluidized bed boiler power         plant;     -   c) Pollution level meeting the most demanding standards despite         the use of low grade fuels because FBB boilers have the unique         ability to effect the combustion of a fuel granulate mixed with         a reagent, often limestone, also as granulate, while in         suspension in the fluidized bed. Thus, it is beneficial to the         invention the ability of FBB to react during combustion the SOx         gases with a reagent (e. g. limestone) mixed with the fuel, the         reaction producing an inert effluent (e. g. solid calcium         sulfate or plaster). Other gases or ashes are adsorbed in this         solid effluent. Also, because of lower combustion temperature,         NOx are not generated. See         -   http://fossil.energy.gov/education/energylessons/coal/coal             cct4.html     -   d) Savings on fuel costs, because the FBB boiler can efficiently         burn highly contaminated fuels (sulfur, metals, salts . . . ) of         low caloric content, which are low in price and are sometimes         disposed of as effluent (highly contaminated petroleum coke for         instance); furthermore, the FBB is very flexible to variations         of fuel quality (heat value, ash content, moisture, sulfur         content, etc.) without interference on efficiency and pollution         See Myöhänen, Lundqvist & Kinnunen (Foster Wheeler), Kral         (Siemens AG): The Advantages of Supercritical Fluidized Bed         Boiler at:         -   http://www.siemenswestinghouse.com/download/pool/steam             turpowplants 04.pdf     -   e) Ability to trade about ¼ to ½ of energy generated to a local         utility at a market price dictated by conditions of high energy         demand, high kWh rates, and energy shortages for a long term         future going with high GDP growth. The investment project will         depend much less on political negotiations regarding procurement         of energy from a dominant utility, often regulated or         state-owned, a situation which always delays projects and         increases their engineering and start-up costs. Contracts with         the utility can be made more flexible thanks to a power plant         using either 2 or 3 boilers, or the ability of a smelter to         operate under its full capacity during peak hours. This means         that, at energy rates fitting with local market conditions, the         plant will offer to the utility typically three energy rates: a         base rate for a nominal consumption level on a “consume or pay”         basis, a regular rate for energy consumed above the nominal         level, and a premium rate for the short peak hours and seasons.     -   f) All this combines into very competitive metal production         cost. As a consequence, a Computer Model (See CD-Rom Appendix         attached) simulating all cost factors in the combination of both         Power Plant and Smelter demonstrates that the marginal energy         rate charged to the smelter can be between 0.8 and 2 US         cents/kWh, and that the smelter operating cost will be at most         $1,100/tonne, putting the new smelter in the first quartile of         world smelters in terms of operating costs in the generally         recognized Commodities Research Unit (CRU) classification.         Meanwhile, the Computer Model (See CD-Rom Appendix attached)         shows that in average market conditions for key resources         produced and consumed, an integrated smelter/power plant located         on a major waterway will produce metal at an operating cost of         at most $1,100/tonne and pay a long term Return On Equity of         over 20% while supplying energy to the local utility at         competitive rates.

SUMMARY

A method producing aluminum combining an aluminum smelter and a power plant which allows lower capital intensity, small total investment, and competitive operating costs.

DRAWINGS—FIGURES

Fig A shows a prebaked anode cell cross section.

Fig B shows a Soderberg cell cross section.

Fig C-a shows a graph of the ratio of primary aluminum to steel prices from 1910 to 2000.

Fig C-b shows a graph of the ratio of primary aluminum to steel prices from 1972 to 2000.

Fig D shows a generic vertical stud Soderberg and circulating fluidized bed boiler facility.

DESCRIPTION OF INVENTION

The invention combines a Vertical Stud Soderberg smelter without anode paste plant (the anode paste is procured on the free market) with a Fluidized Bed Boiler Power Plant burning either coal of any grade or low quality carbon fuels such as coke or even peat or lignite. The invention envisions a combination of a Soderberg smelter system with its own power plant as shown on flowchart figure D.

-   -   (1) Means for static storage for solid fossil fuel in granulate         or powder form.     -   (2) Similar means for storage of a fuel reagent, most often         limestone, which will react with fuel during combustion to form         a solid effluent or possibly a reusable construction material         such as plaster.     -   (3) Means for mixing intimately said fuel and reagent, such as         any conventional powder mixer very generic in heavy processing         industries.     -   (4) Means to generate a fluidized bed or circulating fluidized         bed allowing the mixture to float during combustion, and to         maintain said combustion while heating steam, such as those         supplied by such energy generation engineering firms as Alstom         Power, Foster Wheeler, Bechtel, ABB Power, Hitachi Power,         Austrian Energy Company and several others.     -   (5) Means to convert steam energy into electrical current,         typically a steam turbine of a conventional type as used in the         energy generation industry.     -   (6) Means to condense steam, typically a condenser for recycling         of used steam into water to the boiler.     -   (7) Means to convert mid-Voltage AC current into low Voltage,         high intensity AC current for the electrolytic process,         typically a rectifier such as supplied by General Electric, Fuji         Electric, Cegelec (Alstom), ABB Power, and several others.     -   (8) Means to transform mid-voltage AC current into high Voltage         AC current for distribution of electricity by the utility(ies)         and other institutional energy consumers such as municipalities,         who will be customers of the plant for its extra energy         production.     -   (9) Means for alumina powder storage. Ultrapure alumina powder         is supplied by Alcoa, Billiton, Alcan Jamaica Co., Bauxilum         (Venezuela), and several others.     -   (10) Means for storage of anode paste such as supplied by Elkem         Carbon, Hydro Aluminium and several others. Anode paste is an         industrial intermediate obtained by kneading together pitch and         ground petroleum coke.     -   (11) Means to scrub exhaust gases from the reduction cell,         typically a scrubber containing alumina powder which reacts with         exhaust gases in adsorbing polluting contents notably Chlorine         Fluoride compounds and Lithium gas conpounds, such as supplied         by ABB Flakt, Solios Environment and several others.     -   (12) Means to finish the anode paste mixing as used in all         Soderberg plants.     -   (13) Means for storage of all other raw materials required, such         as refractories, carbon cathode blocks, and numerous other         conventional industrial supplies.     -   (14) Means for storage of specific refractory materials used for         relining of the reduction cells, as used in all aluminum         smelters.     -   (15) Means for alumina reduction applying in a single potline         the Vertical Stud Soderberg design similar to generic designs         existing in such smelters as Sorocaba (Companha Paulista         Aluminio, Brazil), La Coruna (Alcoa Spain), Lista (Elkem), Baie         Comeau (Alcan, Quebec), Aardal (Hydro Aluminium, Norway), and         many others.     -   (16) Means for offline relining of used cells. This shop will be         introduced later, since it is not needed for initial start up.     -   (17) Means for casting molten metal into ingots and sows such as         supplied by Wagstaff Co. (US), Maertz-Gautschi (Austria),         Brochot (France), and many others. 

1. A method for co-producing primary aluminum and electricity, comprising a small scale Soderberg technology aluminum smelter with a solid fuel fired fluidized bed boiler power plant.
 2. The method of claim 1 wherein the relationship between the power produced by the power plant is predetermined according to a cost/benefit scale exposed in computer model described in CD-Rom attached, so that energy traded to local market at market rates more than offsets energy costs, resulting in a low marginal energy rate for the smelting operation.
 3. The method of claim 1 in which low-grade fossil fuel or other solid fuels can be used as the energy source, including municipal waste, etc.
 4. The method of claim 1 in which, optionally, anode paste can be manufactured on site, from the solid fossil fuel, the fuel first being separated by any conventional separation process segregating the 10 percent of the highest purity fuel from the 90 percent of the residual fuel and utilizing the high-purity substrate to produce anode paste instead of procuring it from outside sources. 